The first stages of the growth of a hydrogenated carbon deposit have been investigated by Auger electron spectroscopy on plasma-etched steel substrates. The deposit-to-substrate Auger peak-to-peak intensity curves vs. the quantity of deposited matter are characteristics of the growth mode which is itself very dependent on the plasma cleaning procedure. Substrate cleaning promotes a layer-by-layer growth mode whereas its absence induces an island growth mode. The steel substrate decontamination not only increases the adhesion between the carbonaceous film and the steel surface but also improves the protective properties of the coating. Finally it is shown that on a decontaminated steel sheet, a film of 1.2 nm can be a perfect covering.